In course of syncing files Resilio Sync does a lot of background operations that are hidden and not visible to the user - read, check write files, compare files to those on other peers, etc.
Seeing this error means that Sync has to spend time on some of these operations.
It does not necessarily means that the synchronisation stuck and won't resume. On the contrary, it's intermittent and Sync can recover itself from it. This error is indication that some of the resources - disk, network - may be busy or slow at the moment, or that there are really a lot of files to process at the moment.
However, if that does not happen in the timely manner without apparent reason, please contact support with some details about your setup and debug logs.
The most typical and frequently performed operations are the following
Checking file blocks
Before uploading a file, Sync will check the local copy, read its pieces, and hash them. The receiving peer will check the local file blocks when it needs to download some file pieces. At the same time peers do hashing, and reading file operations.
Copying local file blocks
Relates to deduplication feature and means that peers do not re-download file but instead searches through the local files and copies them, piece by piece. May cause increased disk usage.
Hashing file
Means that peer calculates the file hash. Also, all files are hashed for a newly created job if power user setting lazy_indexing in OFF.
Also, receiving peers will be hashing their local files for pre-seeded folders (apart from Read-Only peers). When files are being hashed it's OK to see disk and CPU usage increase.
Merging folder tree
Each of the peers exchange and compare their files/folders metadata to verify which of them need to be synced. Syncing speed may be slower if Agent performs merge at the moment. Merging may fail if a files or subdirectory cannot be rechecked (for example, folder tree changed during merge ). Agent will attempt to merge it again.
Reading file from disk
Sync reads the file while uploading it to other peers. It will read and hash the file at the same time if lazy_indexing in set to true.
Scanning files
Basically it’s a list of files that will be uploaded marking their size, timestamp, and other attributes. Peer scans the files in folder right after it has been added to Sync (initial folder scan). It will later hash and read files when once it starts uploading them. This operation will performed if new files are added to the folder or during periodic scan.
Transferring
The data transfer itself. Shows the upload and download speed.
Writing file to disk
Peer writes the downloaded or 'deduplicated' file pieces to disk.